In recent years, manufacturers of absorbent pads including sanitary napkins, tampons, diapers, bed pads and the like have been interested in incorporating in or adding to the absorbent elements of such pads, supplementary materials capable of absorbing and immobilizing relatively large amounts of aqueous fluid per unit of absorbent used. One purpose of such additives, of course, is to make the pad more compact, and thus more comformable and less conspicuous in use, while increasing the effective absorbency characteristics. In addition when such supplements are used, the total amount of absorbent material necessary to meet a given capacity requirement is reduced and important economies in production cost can be realized.
Among the supplementary materials suggested for this purpose are the water-insoluble alkali metal carboxylate salts of starch-acrylonitrile graft copolymers obtained by saponifying starch-acrylonitrile graft copolymers with an alkali metal base in an aqueous alcoholic medium. These graft copolymers are characterized as "being water-insoluble granular solids having the ability to absorb water in amount in excess of 50 parts per part thereof while retaining their granular character." A detailed description of such compositions may be found in U.S. Pat. No. 3,661,815 which issued May 9, 1972 to T. Smith and is assigned to Grain Processing Corporation.
While these graft copolymers were indeed found to have an extra large capacity for aqueous fluids when tested as indicated in the patent, it was also found that when the copolymer particles were incorporated in tampon or sanitary napkin structures and tested with aqueous saline fluids formulated to simulate menstrual fluids, that such fluids did not penetrate quickly and the potentially large capacity of the polymers was not utilized. Limited tests with menstrual fluids confirmed these results. Apparently, when viscous menstrual fluids first strike the polymers, the particles tend to expand and agglomerate excessively to form a temporary physical barrier against continued unimpaired flow of fluid into the interior of the pads. As a result the rated performance of these modified napkins and tampons with respect to absorption of menstrual fluids was no better than standard products which did not contain a high absorbency additive.
A further disadvantage was noted with respect to use of these copolymer salts is that they are supplied in the form of a powder having small particle size which tend to dust excessively while the tampons and napkins are being fabricated.
This invention is directed to the discovery that the above indicated disadvantages can be overcome by adding certain materials to these graft copolymers which increases the rate at which they absorb menstrual fluids, while at the same time minimizing the dusting problem.